2乘4分之1+4乘6分之1+6乘8分之1+······46乘48分之1+48乘50分之1
=（1/2-1/4+1/4-1/6+1/6-1/8+……+1/46-1/48+1/48-1/50）÷2
=（1/2-1/50）÷2
=12/25÷2
=6/25